1. Field of the Invention
The present invention relates to moving image processing for frame rate conversion and, more particularly, to a conversion process of performing conversion to a higher frame rate, for example, converting a 60 Hz image into a 120 Hz image.
2. Description of the Related Art
A CRT has long been used as a moving image display device represented by a television receiver, but a panel using a liquid crystal device is recently most frequently used. Features of the liquid crystal device will be explained below with reference to FIG. 10. Referring to FIG. 10, the abscissa indicates the time, and the ordinate indicates the brightness of a pixel. The frame rate is 60 Hz. As shown in FIG. 10, the liquid crystal device holds light emission for 1/60 sec, and hence is called a “hold type” device.
The hold type device has the drawback that blur easily occurs in a motion. FIG. 11 is a view for explaining this drawback. Referring to FIG. 11, the abscissa indicates the position on the screen, and the ordinate indicates the time. FIG. 11 shows an example in which rectangle waves move from the left to the right on the screen. When the eye follows this motion, a state in which a pixel stays in the same position for 1/60 sec in the motion followed by the eye is a relative delay to the motion. If the hold time is long, the delay increases its width and is perceived as a blur of the motion on the screen. A lowermost FIG. 1101 in FIG. 11 shows the way the motion followed by the eye looks, and demonstrates that a blur having a certain width is perceived on each edge.
An example of a measure to counter this motion blur is a method of shortening the hold time by raising the driving frequency. FIG. 12 shows an example in which an image is displayed at 120 Hz as a double frequency. Referring to FIG. 12, the abscissa indicates the time, and the ordinate indicates the brightness of a pixel. As a method of thus doubling the frame rate, a method of displaying an input image by dividing it into an image containing a high-frequency component and an image containing only a low-frequency component in the time direction is known. FIG. 13 shows the dynamic characteristic of an image having undergone double-speed drive performed by this method. Similar to FIG. 11, the abscissa indicates the position on the screen and the ordinate indicates the time in FIG. 13 as well. A lowermost FIG. 1301 in FIG. 13 shows the way a motion followed by the eye looks. Comparison with the FIG. 1101 shown in FIG. 11 reveals that a motion blur is largely reduced in the FIG. 1301 shown in FIG. 13.
Also, as a device having the same light emission characteristic as that of the CRT, a field emission type display device is being extensively developed. FIG. 14 is a view for explaining the light emission characteristic of this device. Similar to FIG. 10, the abscissa indicates the time, and the ordinate indicates the brightness of a pixel. A display device of this type is called an “impulse type” device because the device emits light in a moment of 1/60 sec.
The impulse type device repetitively turns on and off light emission with a period of 1/60 sec, and hence has the drawback that this ON/OFF of light emission is readily perceived as a flicker. Since the flicker becomes conspicuous as the area increases, the flicker often poses a problem in recent display devices having large screens.
FIG. 15 shows the dynamic characteristic of the impulse type device. Referring to FIG. 15, the abscissa indicates the position on the screen, and the ordinate indicates the time. Unlike the characteristic of the hold type device, the most significant feature is that no motion blur perceivable as an afterimage occurs.
It is possible to raise the driving frequency as a measure to cope with the flicker as well. FIG. 16 shows an example in which an image is displayed at 120 Hz as a double frequency. Referring to FIG. 16, the abscissa indicates the time, and the ordinate indicates the brightness of a pixel. In the impulse type device, the brightness of one light emission can be obtained by displaying the half level of the brightness of one light emission twice.
FIG. 17 shows the dynamic characteristic when displaying an input image by dividing it into an image containing a high-frequency component and an image containing only a low-frequency component in the time direction. If the same frame is simply displayed twice, two frames overlap each other. However, since the high-frequency image is displayed only once, only a blur caused by the low-frequency component occurs. This suppresses visual deterioration.
As described above, the method of dividing a frame image into two subframes in accordance with frequency components is effective as the measure to cope with the motion blur in the hold type display device, or as the measure to counter the flicker in the impulse type display device.
Japanese Patent Laid-Open No. 2006-184896 describes a method of implementing hold type double-speed driving. FIG. 18 shows a part of the circuit configuration.
An input frame 1801 as an input image to the circuit is input to a low-pass filter processor 1802, difference detector 1803, and adder 1805. The low-pass filter processor 1802 generates a subframe containing only a low-frequency component of the input frame 1801. The difference detector 1803 detects the difference between the input frame 1801 and the subframe generated by the low-pass filter processor 1802 and containing only the low-frequency component, thereby extracting a high-frequency component. The adder 1805 adds the extracted high-frequency component and input frame 1801. Consequently, a subframe having an emphasized high-frequency component can be obtained. A switching circuit 1806 switches the subframe containing only the low-frequency component and the subframe having the emphasized high-frequency component with a period of 120 Hz, thereby supplying an output frame 1807 to the subsequent processing. The subsequent processing reproduces the original frame image when viewed with a time period of 60 Hz by alternately displaying the subframe from which the high-frequency component is removed and the subframe having the emphasized high-frequency component.